Introduzione al calcolo della forza di punzonatura e del centro di pressione di punzonatura

Tempo di lettura stimato: 13 minuti

punzonatura force and its reduction measures

1. Calculation of oscuramento force

The punching force is exerted by the punch on the sheet during the punching process, and it is one of the important factors for selecting the press and designing the mold. Throughout the blanking process, the size of the blanking force is constantly changing, as shown in Figure 1-1. The OA section in the figure is the elastic deformation stage, and the blanking force on the sheet increases linearly with the downward pressure of the punch. Section AB is the stage of plastic deformation. Point B is the maximum value of the punching force. When the punch is pressed down again, cracks form in the material and expand rapidly, and the punching force decreases, so BC is the fracture stage. When reaching point C, the upper and lower cracks overlap, and the sheet has been separated. The pressure used by the CD is only to overcome the frictional resistance and push out the separated material. Blanking force refers to the maximum resistance of the sheet material on the punch. When the sheet material acts on the punch to produce the maximum resistance and cracks (point B in Figure 1-1), the shear in the shear deformation zone of the sheet material is used as the material’s shear strength (MPa).

Per oscuramento with ordinary flat blades, the blanking force F can be calculated by the following formula.

F=KLtτ_B

In the formula F-punching force;

L—-The length of the punching periphery;

t—-material thickness;

b—-shear strength of material;

K—-Coefficient. The coefficient K is a correction coefficient that takes into account the influence of factors such as the fluctuation and unevenness of the mold gap value, the wear of the cutting edge, the mechanical properties of the sheet, and the thickness fluctuation in the actual production. Generally, take K=1.3.

In general, the tensile strength of the material σ_B=1.3τb. For the convenience of calculation, the punching force can also be calculated by the following formula.

F=Ltσ_B

2. Measures to reduce blanking force

When punching high-strength materials or thick materials and workpieces with large dimensions, the required punching force is larger, which exceeds the nominal pressure of the selected equipment. The following methods are commonly used to reduce the punching force.

• Step punch punzonatura

In a multi-punch mold, different heights can be made according to the size of the punch, so that the working end faces are arranged in a stepped shape. As shown in Figure 1-2.

The force reduction principle of step punch punching is that it prevents several punches from being punched at the same time, avoiding the simultaneous occurrence of the maximum punching force of multiple punches, thus reducing the total punching force.

The height difference H between the punches depends on the material thickness.

Thin material: when t<3 mm, H=t;

Thick material: when t>3 mm, H=0.5 t.

When using a stepped punch, the thin punch should be as short as possible, which is beneficial to its strength; in addition, the punch should be arranged as symmetrically as possible to prevent the deflection of the mold. Step punch punching can reduce the punching force, reduce vibration, without affecting the accuracy of the workpiece, and avoid tilting and breaking of the small punch that is close to the large punch. When all the punches are of the same height, the small punch that is close to the large punch is affected by the material flow caused by the large punch, and it is easy to tilt or break the small punch. The disadvantage of this method is that the long convex mold is inserted deeper into the concave mold, which is easy to wear, and it is troublesome to sharpen the cutting edge. It is mainly used for molds with multiple convex molds and relatively symmetrical positions.

The punching force of the step punch is generally only calculated according to the step that produces the largest punching force.

• Cancellazione with oblique blade

Flat-blade blanking is to simultaneously punch the material along the entire periphery of the cutting edge, so the punching force is relatively large. If the punch (or die) cutting edge plane is made into an inclined plane that is not perpendicular to the direction of movement, the cutting edge will not be in contact with the periphery of the blanking part at the same time during punching, but will gradually cut the material away, which can significantly reduce Punching force.

For punching with oblique blades, in order to obtain flat parts, the punch should be flat blade when blanking, and the concave mold should be oblique blade. When punching, the concave die should be flat blade and the punch should be oblique blade. The oblique blades should also be arranged symmetrically, so as to prevent the die from shifting due to unidirectional lateral pressure during punching and gnawing on the cutting edge. The cutting edge forms of various oblique blades are shown in Figure 1-3.

Figure 1-3 shows the value of the height H of the inclined blade. When the material thickness t<3mm, H=2t; when the material thickness t=3~10mm, H=t.

The calculation formula of the blanking force of the oblique blade is

F _oblique = K _oblique Ltτ

In the formula, F _oblique —- blanking force of oblique blade;

K _oblique —- the parameter of force reduction, its value is related to the height H of the oblique blade. When H=1, K _slope=0.4~0.6; when H=2 t, K _oblique=0.2~0.4.

The advantage of oblique blade blanking is that the press can work under soft conditions. When the blanking parts are large, the force is reduced significantly. The disadvantage is that the mold is complicated to manufacture, the cutting edge is easy to wear, and it is difficult to grind. The blanking parts are not flat enough, and they are not suitable for blanking parts with complex shapes. Therefore, in general, try not to use them, and only use them for large stamping parts or thick plates Blanking.

When using oblique blade punching or step punch punching, although the punching force is reduced, the punch enters the fourth mold deeper, and the punching stroke increases, so these molds save effort and no effort.

• Heat punzonatura (red punching)

Heating blanking is also called red blanking. Metal has a certain shear strength at room temperature, but when the metal material is heated to a certain temperature, its shear strength is significantly reduced, so heating and punching can reduce the punching force (heat the metal material to 700~900 ℃, The punching force is only 1/3 of normal temperature or even less).

The advantage of heating blanking is that the force is reduced significantly, but the disadvantage is that heating is easy to produce hydrogenated skin and damages the surface quality of the workpiece; and because of heating, the working conditions are poor. Heating blanking is generally used for the blanking of thick materials and the blanking of workpieces with low tolerance levels.

3. Calculation of discharge force, thrust force and ejector force

When punching, there is elastic deformation before the material is separated. At the end of punching, due to the elastic recovery of the material and the existence of friction, the blanking parts or punching waste are blocked in the die, and the remaining material is blanked. Hoop tightly on the punch. In order to continue the punching work, the material hooped on the punch must be unloaded, and the material stuck in the die must be pushed out. The force required to unload the hoop material from the punch is called the unloading force F _unloaded; the force that pushes the workpiece or wastes out of the punching direction from the die is called the force F_push. The force required for the workpiece or waste to be ejected against the punching direction is called the ejector force F _top.

It is difficult to accurately calculate these forces. The following empirical formulas are commonly used in production.

F _unload=K _unload F

F push = nK push F

F top=K top

In the formula F—-punching force;

F _unloading, F _pushing, F _top—-unloading force, pushing force, ejecting force;

K _unloading, K _pushing, K _top—-discharge force, pushing force, ejector force coefficient, see Table 1-4;

n—-The number of blanking pieces (or scraps) stuck in the die at the same time.

Material thickness（mm）	K unloading	K pushing	K top
Acciaio	≤0.1	0.06~0.09	0.1	0.14
	>0.1~0.5	0.04~0.07	0.065	0.08
	>0.5~2.5	0.025~0.06	0.05	0.06
	>2.5~6.5	0.02~0.05	0.045	0.05
	>6.5	0.015~0.04	0.025	0.03
Red copper	Ottone	0.02~0.06	0.03~0.09	0.03~0.09
Alluminio	Lega di alluminio	0.03~0.08	0.03~0.07	0.03~0.07

Note: Unloading force coefficient K is used when unloading holes, large edges, and complex contours, take the upper limit.

n=h/t

In the formula, h—-the height of the straight edge wall of the cavity of the cavity;

t—-The thickness of the sheet.

4. Determination of the nominal pressure of the press

The unloading force, pushing force, and ejecting force are transmitted by the press and the mold unloading device or ejecting device. Therefore, when selecting the nominal pressure of the equipment or designing the die, it should be considered separately.

When punching, the nominal pressure of the press must be greater than or equal to the sum of the various punching process forces F_total. The total calculation of F should be treated separately according to different mold structures.

When using the elastic pressure unloading device and the mold with the lower discharge method,

F _total==F+F _unloading+F _pushing

When using the elastic pressure unloading device and the mold with the upper discharge method,

F _total==F+F _unloading+F _pushing

When using a rigid discharge device and a mold with a lower discharge method,

F _total = F + F _push

Calculation of oscuramento pressure center

The pressure center of the mold is the point of action of the resultant force of the pressing force. The pressure center of the mold must coincide with the centerline of the pressure slider through the axis of the mold handle. Otherwise, the slider will be subjected to eccentric load during stamping, resulting in abnormal wear of the slider guide rail and mold guide part, and the reasonable gap will not be guaranteed, which will affect the quality of the parts and reduce the life of the mold, and even damage the mold.

1. Determining the pressure center of simple geometric figures

• The pressure center of the straight line is located at the center of the straight line.

• The pressure center of the symmetrical blanking part is located on the geometric center of the contour figure of the blanking part.

• When punching the arc line segment, the position of the pressure center, as shown in Figure 1-5, is calculated by the following formula.

X_o =180Rsina/πa =Rb/l

Here l—-arc length.

The meanings of other symbols are shown in Figure 1-5.

2. Determination of the pressure center of the multi-punch mold

To determine the pressure center of a multi-punch mold is to determine the pressure center of each punch and then calculate the pressure center of the mold. Figure 1-6 shows the position distribution of punches for punching multiple holes. The steps to calculate the center of pressure are as follows.

• Draw the position of the contour of each punch’s edge according to the scale.

• Draw the coordinate axis x, y at any position. When selecting the position of the coordinate axis, try to take the coordinate origin as the pressure center of a certain edge contour, or make the coordinate axis pass through the pressure center of the punch edge contour as much as possible. The coordinate origin should preferably be several punch edges. The center of symmetry of the pressure center of the mouth profile, which can simplify the problem.

• Calculate the pressure center and coordinate positions x₁, x₂…x_n and y₁, y₂…y_n of the punch edge contour respectively.
• Calculate the punching force F₁, F₂…F_n of the punch edge contour and the perimeter L₁, L₂…L_n of each punch edge contour respectively.

F₁=KL₁T_B

F₂=KL₂T_B

…

F_n=KL_nT_B

For the parallel force system, the resultant force of the blanking force is equal to the algebraic sum of the forces. That is, F=F₁+F₂+…+F_n.

Secondo il teorema della meccanica, il momento della forza risultante su un certo asse è uguale alla somma algebrica delle forze componenti sul momento coassiale, quindi si può ottenere la formula di calcolo della coordinata del centro di pressione.

Sostituto F₁, F₂…F_n rispettivamente nella formula precedente, diventano le coordinate del centro di pressione

3. Determinazione del centro di pressione dello stampo di parti di forma complessa

quando punzonatura parti di forma complessa, il principio di calcolo del centro di pressione dello stampo è lo stesso di quello del centro di pressione di punzonatura multipunzone, come mostrato nella Figura 1-7. I passaggi specifici sono i seguenti.

• Selezionare l'asse delle coordinate x e l'asse delle coordinate y in qualsiasi punto del contorno del bordo.

• Dividi la linea di contorno del tagliente in un numero di segmenti di linea semplici in base agli elementi di base e trova la lunghezza di ciascun segmento di linea L₁, L₂…L_n.

• Determina la posizione del baricentro x₁, x₂…x_n and y₁, y₂…y_n di ogni segmento di linea.

• Calcola le coordinate del centro di pressione (x₀, y₀) del profilo del bordo secondo la formula.

Per determinare il centro di pressione dello stampo di tranciatura, oltre al metodo analitico sopra menzionato, può essere utilizzato anche come metodo di imbutitura e metodo di sospensione.

Il metodo di disegno è lo stesso del metodo analitico. Può trovare il centro di pressione della punzonatura con più punzoni e il centro di pressione della punzonatura di parti di forma complessa. Tuttavia, la precisione del metodo di disegno non è elevata e il metodo non è semplice, quindi Soggetto a determinate restrizioni nell'applicazione.

Nella produzione, il metodo di sospensione viene spesso utilizzato per determinare il centro di pressione di parti di tranciatura complesse. Utilizzare fili metallici sottili uniformi per piegarsi lungo il contorno di punzonatura per formare una parte simulata, quindi appendere la parte simulata con filo per cucire e tracciare un filo a piombo dal punto di sospensione; quindi prendi un altro punto della parte simulata e disegna un'altra parte, allo stesso modo, una linea verticale, l'intersezione delle due linee verticali nel centro di pressione. La base teorica del metodo di sospensione è di sostituire la forza di tranciatura uniformemente distribuita sul contorno del oscuramento parte con un filo metallico omogeneo e il baricentro della parte simulata è il centro di pressione della tranciatura.